First: Juice it up. Cut and squeeze a lemon, lime or orange into a glass of water and drink it. You'll get up to 100 extra milligrams of vitamin C, and you'll prevent the dehydration that leads to fatigue. If you need more of a lift, stop and smell the fruit. Studies show that just sniffing citrus improves your mood.
Next: Touch your toes for several seconds. This increases the blood flow to your head, which gives you a quick boost of energy. Backbends are also great. When you stretch your spine you stimulate your nervous system. And if you're really ambitious, stand on your head. You'll stir up feel good hormones, like serotonin, that cause you to feel happy and energetic.
And one last energy-booster: Audition for American Idol! Pop in a CD and belt out your favorite song. Not only will you entertain yourself and probably those around you but the extra oxygen intake will put a momentary lift in your spirits.